Color display panels are typically constructed by locating differently colored sub-pixels at each pixel of the display panel. Color is provided by a color filter layer having color filter elements aligned with light valves that regulate the amount of light passing through each sub-pixel. The overall color and brightness of the light associated with a given display pixel location is perceived by the human eye as a mixture of the differently-colored sub-pixels at that location. To prevent cross contamination of light between sub-pixels (i.e., light following paths through the light valve of one sub-pixel and the color filter element of a neighboring sub-pixel) and resulting loss of blackness and color saturation, a so-called black mask layer provides masking at the boundaries between the color filter elements.
The light valves at each sub-pixel are typically provided by an electro-optic or magneto-optic (“EO/MO”) display. These include liquid crystal, electrophoretic, cholesteric, and Gyricon displays (as discussed in my U.S. application Ser. No. 09/882,311, filed on Jun. 15, 2001). Usually the light valves all generate the same color light, and it is the color filter elements that produce the color of the display.
Ideally, a color filter element and surrounding black mask would be at the same location along the normal direction of the display (i.e., the direction normal to the plane of the display). In other words, there would ideally be zero space along the normal direction between the light valves and the associated color filter elements and black masks. This would minimize or eliminate the possibility of cross contamination of light between sub-pixels.
In an electro-optic and magneto-optic display, the light valve is typically located in a very thin gap between two substrates. For example, in a liquid crystal display, the liquid crystal material is what acts as the light valve, and it is sandwiched in a very thin layer between two substrates. To minimize color inaccuracy, the prior art has conventionally placed the color filter and black mask layers in that same thin gap between the substrates (e.g., in FIG. 1 the color filter and black mask layers have been deposited on the internal surface of one of the substrates). Materials have been selected that can withstand the harsh manufacturing processes followed in manufacturing an EO/MO display without detrimental impacts on the performance of the color and black mask layers. Typical black mask materials are black organic color agents or thin metal films. Metal films are generally favored over organic materials because of their superior light-intercepting performance.
As noted in U.S. Pat. No. 5,399,374, one of the methods for forming the color filter with a black mask composed of a thin metal film is to use an etching process. First, a conductive film such as indium tin oxide (ITO) which can be etched is formed on a transparent substrate such as a glass, and then the conductive film is etched to be formed into the configuration of the black mask having a predetermined pattern. After that the black mask is formed on the conductive film by performing electroless plating using a metal such as nickel. Color patterns are then laminated on the black mask. Another method for forming the color filter is to first sputter a metal such as chromium on a transparent substrate to form a thin film of the metal, and then to etch that film into the configuration of the desired black mask pattern. Patterned color filter layers are then laminated over the black mask layer. According to still another method, a resist is applied to areas of a transparent substrate that are not to receive the black mask, a metal is sputtered into a thin film over the resist, and the resist is removed, leaving the black mask in areas not originally covered by the resist.
In some prior art, the color filter and black mask layers have been located outside of the display cell, but this results in parallax problems, as illustrated in FIG. 2. In the figure, the color filter and black mask layers have been applied to the outer surface of one of the display cell substrates. Three differently-colored sub-pixels are shown in the figure, each with its own color filter element, and with black masks located between the filter elements (in a layer just above the color filter layer). The light valve for each sub-pixel is provided by the liquid crystal layer between the substrates, and is spaced above the color filter and black mask layers by at least the thickness of the substrate. Light traveling through the display along paths 100 fairly close to the normal direction is not particularly affected by moving the color filter and black mask layers outside the substrates, but light traveling along paths 102, 104, at incident angles a substantially away from the normal direction (0° incident angle being the normal direction) can produce inaccuracies, because such angled light can pass through the color filter of one sub-pixel but through the light valve of an adjoining, and differently-colored, sub-pixel.
The parallax problems caused by the separation of the color filter and black mask from the light valve are of at least two different types. First, is a loss of blackness. This is illustrated by light traveling along path 102 in FIG. 2. In this example, the leftmost sub-pixel is supposed to be turned off, i.e., the color component corresponding to that sub-pixel is supposed to be zero. If it were not for the parallax difficulty, the display would appear dark over the leftmost sub-pixel. The problem is that light traveling along path 102, which would have been blocked by a black mask had the black mask layer been located at its conventional location (in white areas 42), is able instead to pass through the display. This has the undesirable effect of reducing the blackness of a display, i.e., the extent to which a desired sub-pixel or group of sub-pixels or entire pixels can be shut off entirely.
Another problem that parallax causes is loss of color saturation and color shift. This is illustrated by light traveling along path 104 in FIG. 2. The color and intensity (saturation) of color associated with the leftmost sub-pixel should be prescribed entirely by the leftmost color filter and the leftmost light valve, respectively. But light traveling along path 104, though it has its color prescribed correctly by the leftmost color filter, it has the intensity of that color prescribed by the light valve associated with the neighboring sub-pixel.
The table of FIG. 3 compiles the color saturation, brightness, and blackness (in percent) for light incident at 30° for different substrate thicknesses (1 mil, 2 mil, and 3 mil) and different spacings between the color filters. The graphs of FIGS. 4-6 compile the same three parameters (color saturation, brightness, and blackness, in percent) for a 10 μm color filter spacing for different light incident angles at the same three substrate thicknesses. The color filter spacing is the width of the black mask dividing adjoining color filter elements. As can be seen in the table and graphs, when the spaces between the color filters decreases, the brightness of the display increases and the color saturation decreases slightly. When the spaces between the color filters increases, the color saturation increases but the brightness decreases quite dramatically. As the thickness of the plastic decreases, the color saturation improves but the brightness stays constant. Blackness remains essentially unchanged at all of the parameter settings.
As noted earlier, the conventional solution to this parallax problem is to fabricate the color filter and black mask layers on the inner surface of one of the substrates, thus placing it within a few microns of the optically active element (e.g., the liquid crystal light valve). The difficulty with this method is that the color filter, when it is placed inside the display, must undergo all of the harsh processing necessary to manufacturing the display cell. The color filter is generally placed underneath both the ITO layer and the polyimide alignment layer (as the ITO layer needs to be as close to the liquid crystal as possible to reduce drive voltages, and the alignment layer needs to be in direct contact with the liquid crystal to provide the alignment). As a result, the color filter must withstand all the processing required to produce the ITO and alignment layer. These include harsh acids, bases, high temperatures (greater than 180° C.), and solvents. This requirement severely limits the chemicals available to provide the color, adding cost and reducing design flexibility and product performance.
As noted, there are instances in the prior art in which the color filter and black mask layers have been moved outside of the display cell. But none of these has addressed, let alone solved, the parallax problem. For example, U.S. Pat. Nos. 4,877,697, 4,610,508, and 4,673,253 suggest that displays be constructed by exposing color film while the film is in registry with the display element, to reduce registration errors between the color filter elements and the electrodes that define the locations of the light valves of the display. U.S. Pat. No. 5,754,261 shows the color filter layer located on the outside surface of the substrates. U.S. Pat. Nos. 4,690,511 and 4,560,241 show using a very thin auxiliary layer of glass dividing the color filter and the liquid crystal layer. U.S. Pat. No. 4,953,952 shows the color filter placed inside a plastic film laminate structure of the polarizer filter.